1. Field of the Invention
The present invention relates to a method for shaping a sheet blank made of a material having an exponential stress-strain behavior and to an apparatus suitable for performing this method.
2. Description of Related Art
In the space and aircraft industry, titanium and its alloys are used more and more for fuel containers and the like due to the low weight and the good wear resistance. The titanium-.beta.-alloys especially suitable for this purpose however may be cold-formed unsatisfactorily only. These alloys have an exponential stress-strain behavior, as illustrated in the schematical stress-strain diagram in FIG. 1. The diagram shows that the titanium-.beta.-alloys do not have the usual strain hardening behavior, so that during the tensile test at room temperature above the apparent yield point, in the range of plastic deformation, an indentation and then the rupture takes place without a further increase of the stress. This has a great effect on the cold-formability of these materials. At a very low percental cold forming the danger may occur that either fatigue ruptures occur or that the material dents in an uncontrolled manner in case the material is not subjected to a heat treatment after every minor shaping step. The most unproblematic cold forming method is the cold rolling process, by means of which only planar sheets may be manufactured.
In particular shells with a greater diameter (more than 60 mm), a small wall thickness (below 3 mm) and/or a high bulging (semi-sphere) have been manufactured up to now by hot shaping methods, wherein afterwards the desired wall thickness had to be achieved by machining.
At higher temperatures, titanium and its alloys have a higher affinity towards air components, by which on one hand a corrosion layer forms at the surface of the workpiece and by which on the other hand the material becomes brittle by hydrogen absorption. Both are highly undesired and can only be avoided or eliminated if either the heating up (for hot shaping or for the heat treatment) is carried out in a protective gas atmosphere or if the corroded layer is removed mechanically or if the brittleness is eliminated by heat treatment). It is already known from U.S. Pat. No. 3,815,395 to form tank bottoms by means of two spinning rolls engaging opposite sides of the workpiece. In the apparatus described in this U.S. Patent, the workpiece is clamped centrally and supported freely movably, whereas the spinning rolls are rotationally driven and are also guided via a predetermined, radial path (by means of which the workpiece rotates). By means of this superposition of the movement control, it cannot be avoided that local tensile stresses appear. According thereto, the apparatus is either suitable for a hot-forming process of for workpieces with a normal cold hardening behavior.
It is the object of the invention to provide a simple and inexpensive method and an apparatus for cold forming a material with an exponential stress-strain behavior to form hollow shells of a small wall thickness.